In the production of cement clinker, approximately 0.53 t CO2/t clinker results from the deacidification of the limestone and approximately 0.28 t CO2/t clinker from the combustion of fuel in the burning process. Those quantities of carbon dioxide (0.81 t CO2/t clinker) have hitherto been emitted into the atmosphere by way of the exhaust gas, although the effect of carbon dioxide as a greenhouse gas is generally known. Efforts are therefore being made to reduce emissions drastically in future.
Against that background, methods that permit separation of CO2 from the exhaust gases of combustion processes are currently being discussed.
DE 196 37 320 A1 discloses a method for producing cement in which the raw meal is preheated, calcined and finally burnt in a rotary tubular kiln. In that method, the raw meal is deacidified in a calciner through which kiln exhaust gas flows. The calciner operates in accordance with the entrained flow principle. Before being charged into the rotary tubular kiln, the calcined raw meal is delivered to a suspension reactor having a circulating fluidised bed. The fluidised bed is formed with heated air in which the calcined material is heated to temperatures of from 1000 to 1200° C., so that impurities are expelled.
In order to reduce CO2 emissions, in particular the so-called oxyfuel process is of interest for cement production. In that process, the fuel is reacted with almost pure oxygen, so that the exhaust combustion gas does not contain nitrogen and consists almost entirely of CO2 and water.
WO 2008/059378 describes such a method in which the exhaust gas of the calciner is concentrated with regard to the CO2 content to such an extent that it can be delivered to a storage stage. Thus, approximately 75% of the carbon dioxide generated during clinker burning can be separated off, without the need for any appreciable modifications to the clinker production process. The calciners used, as also in WO 2008/059378, are usually so-called entrained flow calciners, in the case of which the raw meal to be treated is transported by a carrier gas (usually the exhaust gas of the kiln or tertiary air) through the calciner, while the heat treatment takes place. If, however, the calciner is no longer operated with the kiln exhaust gas or the tertiary air of the cooler, but with oxygen, the amount of exhaust gas is greatly reduced. In WO 2008/059378, that problem is solved by raising the quantity of carrier gas by returning the exhaust gases of the calciner. Owing to the high gas temperatures of the recirculated exhaust gas, it is necessary, for the operation of a fluid-flow machine (fan), for the gas to be cooled by means of heat exchangers or to be quenched with a gas of lower temperature, as proposed in WO 2008/059378. As a result of the recirculation of some of the exhaust gases of the calciner, local over-heating in the oxygen-operated calciner can also be avoided.
Recirculation has, however, the disadvantage of recarbonation, that is to say, the re-formation of CaCO3 from CaO and CO2. Owing to the high CO2 partial pressure in the oxyfuel process, recarbonation takes place at a high reaction rate when a cooling of the material to be burnt to temperatures below the equilibrium temperature of approximately 850° C. takes place. For the process, that operation results in an increased thermal energy requirement for the calciner since the recarbonated material has to be deacidified again.